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From "Alec Matusis" <matu...@yahoo.com>
Subject [users@httpd] requests time-out under load, no warnings in logs
Date Sun, 30 Sep 2007 22:54:59 GMT
We are running a busy Apache/2.0.54 server on 2.6.9 kernel, that suddenly
becomes very slow- requests either time out, or it takes 10s to serve a 1K
thumbnail. 
It is usually correlated with load spikes, but not perfectly (by looking at
the bandwidth graph)
Then it requires a stop/start, and it becomes fast again.
This has been happening before, and it was always accompanied with 

[error] server reached MaxClients setting, consider raising the MaxClients
setting 

in the apache log.

We raised MaxClients    from 200 to 300, and now it happens with no warning
in any logs at all, except with the usual

web10 kernel: possible SYN flooding on port 80. Sending cookies. in
/var/log/messages (this has always happened when apache hung from overload).

What worries me is that there are no log messages in apache logs at all when
this happens.

This is the relevant part of httpd.conf

ServerLimit 300
# we are using prefork MPM
<IfModule prefork.c>
StartServers          10
MinSpareServers       5
MaxSpareServers       20
MaxClients            300
MaxRequestsPerChild   10000
MaxMemFree            2560
</IfModule>

The server has 4GB of RAM and 4GB of swap. When apache "crashes" i.e.
becomes very slow, the swap size is still 0 and vmstat shows no swapping.
I suspect the problem may be in

MaxMemFree   2560

but then I would expect some kind of "out of memory" errors in the logs?
Another question: what is the good choice of MaxMemFree value? We want to
leave 512MB-1GB or RAM on this machine for other tasks.





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